HF geolocation is challenging compared to VHF or UHF geo-location due to the fact that HF signals refract off the ionosphere. The refracted HF signal, called a sky wave, behaves in a way that means the employment of geolocation techniques becomes significantly more complicated. Typical TDOA and FDOA geolocation techniques assume a direct LOS (direct wave) between the transmitter and the receiver. This assumption allows a line to be traced across the surface of the Earth (isochrones and/or isodops) that represents all the possible emitter locations when two spatially separated receivers measure a given TDOA or FDOA.
However, for a signal that has “bounced” off the ionosphere, the ground, or both, the assumptions and techniques employed for direct wave geolocation are invalidated. Thus, the ability to use the sky waves to perform geolocation, and therefore also the ability to determine the location of a target that is over the horizon, is currently severely limited. Moreover, even though there may be relatively reliable mechanisms to perform geolocation on direct wave signals, even this form of geolocation is somewhat questionable when it is not clear whether the emitter is providing direct wave or sky wave signals to the receiver.
Thus, the ability to use sky wave signals for geolocation would be valuable both when direct wave signals are not available, when both direct wave and sky wave signals are available, and when it is unclear as to whether a received signal is a direct wave or a sky wave.
Accordingly, it may be desirable to define ways to use sky wave signals for geolocation.